Workpiece clamp with two alternately applicable compression rings

ABSTRACT

A workpiece clamp ( 1 ) for a machine tool that drills at high speeds is provided with two compression rings ( 2, 3 ) with holes ( 2   f   , 3   f ) of different diameters for the passage of drilling tools of different diameters. The rings are mounted on the clamp, symmetrically and on opposite sides of the drilling axis (Z-Z) in such a way that they are able to rotate about the horizontal axes of two pinions ( 4, 5 ) meshing with a rack ( 6 ) that is moved in both directions relative to a central point (P) through a stroke (C) by a double-acting pneumatic piston ( 7 ). The form, dimensions and positions of the rings are such that, in response to the rotation of their respective pinions only one, alternatively, of the said rings is placed in the working position, with the axis of its hole coincident with the drilling axis (Z-Z); the other ring being laterally displaced where it does not interfere with the drilling axis (Z-Z).

The present invention relates to machine tools that use spindles fordrilling at high speeds.

More particularly, it relates to spindles themselves, and morespecifically the workpiece clamps which they usually have, which fulfilthe function of pressing down on the workpiece being drilled via acompression ring or “foot”, to keep it still and compressed againstother workpieces beneath it to obtain precise and exactly aligneddrilling, and ensure that chips cannot get between the adjacent faces ofseveral superimposed workpieces.

To perform the said function, the said compression ring has a hole whoseaxis coincides during the machining action with the drilling axis, inother words with the axis of the tool for doing the drilling.

Particularly when drilling several superimposed sheets of syntheticmaterial, such as those that make up “multilayer” printed circuitboards, it is necessary for the said hole of the compression ring tohave as small a diameter as possible in order to improve the efficiencyof the chip removal system and ensure that the elastic recovery of thematerial through it cannot be a source of inaccuracies in the machiningor allow the abovementioned intrusion of chips between the superimposedlayers.

It is for this kind of reason that, when holes of different diametersare to be drilled in sequence in the same workpiece, compression ringswith holes of different diameters are used, being changed as requiredduring machining.

Changing the rings at present requires manual disassembly and reassemblyof the rings, fetching them from a magazine which may be separate fromthe machine tool or provided in a suitable position on the machine toolitself. This operation wastes a lot of time and, even when usingsemiautomatic devices for changing the rings, it brings about anappreciable rise in the production costs.

Since it is usually sufficient to have two compression rings withdifferent diameter holes, especially when drilling printed circuitboards, it seemed to the inventor of the present innovation that thedowntimes and constructional complications described above could beeliminated by providing a workpiece clamp on which two compression ringsare mounted, with mechanical means for positioning them alternately, inaccordance with the machine tool programme, in the working position, bya simple, fast operation.

To this end the inventor has provided a solution whereby a rack, movedby a double-acting pneumatic piston, causes by its movement asimultaneous rotation of two pinions integral with the two compressionrings, the effect being to position each alternately in its workingposition, and the other in a position where it cannot interfere with thetool while the tool is working.

To this end he devised the subject of the present invention, which is aworkpiece clamp as described in the appended claim 1.

A more detailed description will now be given of a preferredillustrative embodiment of the workpiece clamp of the invention: in thecourse of this description reference will also be made to the appendeddrawings, which show:

-   -   in FIG. 1 the said illustrative embodiment of the workpiece        clamp of the invention in a section taken on parallel vertical        planes, in the situation in which one of the two compression        rings is in the working position, i.e. with the axis of its hole        coinciding with the drilling axis;    -   in FIG. 2 the same section as in FIG. 1 with the workpiece clamp        in the waiting position and neither of the two rings in the        working position;    -   in FIG. 3 the same section as in the previous figures, in the        situation in which the compression ring that was in the working        position in FIG. 1 has been replaced by the other compression        ring;    -   in FIG. 4 a cross section through the workpiece clamp in the        same situation as in FIG. 1, clearly showing the position of the        drilling tool relative to the double-acting piston by which the        compression ring is moved.

To begin with FIG. 1, this shows that a workpiece clamp 1 according tothe invention is fitted with two compression rings 2, 3 each having ahole 2 f, 3 f for the tool to pass through (the tool being shown only inFIG. 4). These compression rings are connected rigidly, on oppositesides of the drilling axis Z-Z of the tool 12 (see also the said FIG.4), to two pinions 4, 5 in such a way that they rotate with them abouttheir axes, which are horizontally parallel and perpendicular to theplane of the drawing.

The abovementioned pinions 4, 5 mesh with a horizontal rack 6 connectedby a link rod 8 to a double-acting pneumatic piston 7 which can executea stroke C in both directions S, T (see also FIG. 3 for this) from acentral point P situated on the centre plane of the workpiece clamp 1.

The double-acting pneumatic piston 7 (which will be referred to from nowon as the “piston”), of a type known per se, is operated by air flowscontrolled by a system of valves 11, also of known type, in order tocause the rack 6 to execute the said stroke C in both said directions S,T, in such a way as to be synchronized with the machining program.

When machining is not being performed, the workpiece clamp 1, as shownFIG. 2, is in a waiting position, with the piston 7 held in position bytwo identical springs 9, 10 arranged symmetrically in the position inwhich the longitudinal axis of the said link rod 8, which projectsperpendicularly downwards from it, passes through the said central pointP.

In this situation, that is in the situation in which no pneumatic actionis being exerted on the piston 7, the two compression rings 2, 3 (whichfrom now on will be termed “rings” for short) are in a symmetricalposition in which neither of them interferes with the working area, i.e.in which they are at a convenient distance from the drilling axis Z-Z.

When air is introduced into the piston 7 on the right-hand side in thedrawing, as shown in FIG. 1, the rack 6 executes a stroke C in thedirection indicated by the arrow S, rotating both the pinions 4, 5 andthe rings 2, 3 which are connected to them through the same angle aseach other. As the rings are mounted symmetrically, at the end of thestroke C in the said direction S they are positioned as shown, that isone ring 2 has the axis M-M of its hole 2 f coincident with the drillingaxis Z-Z, and the other ring 3 is moved away to the side so as not tointerfere with the machining area.

When the direction of the air flow is reversed, i.e. when acting fromleft to right on the piston 7 as indicated by the arrow T in FIG. 3, thepiston 7, and all the parts whose movement depend on it, move first tothe position shown in FIG. 2 and then on to the position in FIG. 3, thatis where the ring 3, which in FIG. 1 was away from the working area, hasthe axis N-N of its hole 3 f coincident with the drilling axis Z-Z, andwhere the other ring 2, which in FIG. 1 was in the working position, hasbeen moved a suitable distance away. By cyclically repeating theseoperations, synchronized as already described, the two rings can bechanged very fast and completely automatically, thus achieving theresult sought by the inventor.

Obviously, in order for the workpiece clamp 1 of the invention to beable to work, the axes M-M, N-N of the two rings 2, 3 must lie on aplane which does not interfere with the region of the piston 7 andcontains the drilling axis Z-Z. One possible positioning of the partsthat is suitable for this purpose is that shown in FIG. 4, which relatesto the illustrative example described above. In this figure the verticalplane a passing through the axis of the piston 7 is at a distance D fromthe plane β which is parallel to it and contains the drilling axis Z-Z,that is the axis of the tool 12.

1. Workpiece clamp (1) for a machine tool that drills at high speeds,provided with two compression rings (2, 3) with holes (2 f, 3 f) ofdifferent diameters for the passage of drilling tools of differentdiameters, the clamps being characterized in that both of the saidcompressions rings (2, 3) are mounted on the said workpiece clamp (1),symmetrically and on opposite sides of the drilling axis (Z-Z) and insuch a way that they are able to rotate about the horizontal axes of twopinions (4, 5) connected rigidly to the said rings (2, 3), these pinionsmeshing with a rack (6) that is moved in both directions relative to acentral point (P) through a stroke (C) by a double-acting pneumaticpiston (7), the form, dimensions and positions of the rings (2, 3) beingsuch that, in response to the rotation of their respective pinions (4,5), caused by the rack (6) executing the said stroke (C) in one of thetwo directions, only one, alternately, of the said rings is placed inthe working position, with the axis of its hole coincident with thedrilling axis (Z-Z), and the other is placed in a laterally displacedposition where it does not interfere with the said drilling axis (Z-Z).2. Workpiece clamp according to claim 1, in which the said double-actingpneumatic piston (7) is connected to the said rack (6) by a link rod (8)extending perpendicularly downwards in a position corresponding to thesaid central point (P) which marks the start of the said strokes (C) inboth directions, two identical and symmetrically aligned springs (9, 10)being connected to the two opposite sides of the said pneumatic piston(7) such that, in the absence of any pneumatic action, they keep theabovementioned link rod (8) in position with its longitudinal axispassing through the abovementioned central point (P).